/Engine/Source/Runtime/HeadMountedDisplay/Public/IHeadMountedDisplay.h
Returns true, if the App is using VR focus. This means that the app may handle lifecycle events differently from the regular desktop apps. In this case, FCoreDelegates::ApplicationWillEnterBackgroundDelegate and FCoreDelegates::ApplicationHasEnteredForegroundDelegate reflect the state of VR focus (either the app should be rendered in HMD or not).
virtual bool DoesAppUseVRFocus() const